I have seen this happen before. I used a liberal amount of dishwashing
soap mixed with water as a temporary coolant mixture for about a half
hour of idling. Then drain and flush with clear water thoughly before
refilling.

P.S. Always remember to use distilled water in your anti-freeze
solution to keep the mineral content down to a minimum. This works
wonders in eliminating lime buildup in the radiator that eventually
causes overheating. It also keeps the heater core clean inside for
maximum heat transfer on cold winter days.

www.telstar-electronics.com